The temperature within a vehicle parked for a long time in bright sun, particularly in summer, becomes very high, and in some cases above 100.degree. C. The air conditioning system of such a parked vehicle takes a long time before achieving its cooling effect and, as such, the interior of the vehicle is quite inhospitable for the driver and passengers.
To ventilate a parked vehicle, various types of ventilation systems having solar operated Ventilation fans have been proposed in the prior art. One type of solar powered ventilation system is known from, for example, Japanese Patent Publication No. 59(1984)-51,451, entitled "Ventilation Device For Automotive Vehicle," issued on Dec. 14, 1984. This ventilation device, as is schematically illustrated in FIG. 1, has a ventilation fan e disposed in a duct with its air inlet f opening inside the vehicle a and its air outlet g opening outside the vehicle a. The ventilation fan e is connected in series to a solar cell h attached to a rear window glass b as a power source by a bimetal switch i disposed in the vehicle a. This bimetal switch i has a switching temperature set to a specific temperature. The ventilation fan e is, accordingly, automatically operated to discharge air within the vehicle when the temperature within the parked vehicle reaches the specific temperature, i.e., the switching temperature of the bimetal switch.
However, in such a known solar powered ventilation system, since the detection of specific fan actuation temperature is effected independently of the temperature outside the vehicle, the ventilation fan works in response to the temperature inside the vehicle only. Therefore, if the switching temperature of the bimetal switch is set lower than temperatures at which a driver or passengers feel comfortable in the vehicle, the ventilation fan will be actuated even in circumstances in which it is not necessary to ventilate the vehicle, because the temperature in the vehicle is higher than the specific temperature. Such a circumstance will often be present when the temperature inside the vehicle is higher than the specific temperature but the temperature outside the vehicle is too low for comfortable ventilation.